Recuperative furnaces

ABSTRACT

Recuperative furnaces, such as soaking pits, have recuperators formed of sets of ceramic tiles arranged end to end with aligned holes in each set of tiles forming one series of passages for the flue gas and openings between the sets of tiles forming a second series of passages for the air to be heated. The heated air from the recuperator is directed through an air tunnel to an entry or burner port where it is mixed with coke oven gas or other fuel. Combustion is completed in the combustion or heating chamber which also receives the ingots to be heated. The burnt gases pass from the chamber through a flue gas port to the recuperator. It is impossible to maintain a gas tight connection between the tile. To prevent air leakage into the flue gas passages and increase fuel efficiency and production the entry port area is made at least approximately 75 percent (preferably 100 percent) of the cross sectional area of the air tunnel and the flue port area is made at least approximately 75 percent (preferably 100 percent) of the total cross sectional area of the flue gas passages in the recuperator.

United States Patent Shannon et al.

1451 Sept. 12, 1972 [s41 RECUPERATIVE FURNACES [72] Inventors: Robert A. Shannon, Avon Lake; Charles A. Waters, Lorain, both of Ohio [73] Assignee: United States Steel Corporation 221 Filed: Dec. 3, 1 970 21 Appl. No; 94,735

52 us. Cl. ..266/5 s, 263/15 R 51 int. cl. ..C2ld l/06 [58] Field of Search ..263/15 R; 266/5 R, 5 s

[56] References Cited UNITED STATES PATENTS 1,824,876 9/1931 Culbertson .L ..263/15 R 5 2,079,560 5/1937 Morton et al. ..263/15 R 2,092,402 9/1937 Morton et al ..263/51 x 2,124,888 7/1938 Morton et al. ..266/5 R x 2,126,095 8/1938 Dean ..266/5 s x 2,297,696 10/1942 Elder et al ..266/5 8 ux 2,414,888 1/1947 Morton et al. ..263/l5 R 2,563,683 8/1951 Lewis ..266/5 8 UX 3,198,855 8/1965 Suydam ..263/43 x Primary Examiner-4. Spencer Overholser Assistant Examiner-John E. Roethel Attorney-Martin J. Carroll [57] ABSTRACT Recuperative fumaces, such as soaking pits, have recuperators formed of sets of ceramic tiles arranged end to end with aligned holes in each set of tiles forming one series of passages for the flue gas and openings between the sets of tiles forming a second series of passages for the air to be heated. The heated air from the recuperator is directed through an air tunnel to an entry or burner port where it is mixed with coke oven gas or other fuel. Combustion is completed in the combustion or heating chamber which also receives the ingots to be heated. The burrlt gases pass from the chamber through a flue gas port to the recuperator. It is impossible to maintain a gas tight connection between the tile. To prevent air leakage into the flue gas passages and increase fuel efficiency and production the entry port area is made at least approximately 75 percent (preferably 100 percent) of the cross sec'- tional area of the air tunnel and the flue port area is made at least approximately 75 percent (preferably 100 percent) of the total cross sectional area of the flue gas passages in the recuperator.

10 Claims, 5 Drawing Figures PATENTEDSEPIZ 1m 3.660.636

SHEET 1 BF 2 FIG.

INVENTOHS. ROBERT A. SHANNON 8 CHARLES A. WATERS Altar/16y RECUPERATIVE FURNACES This invention relates to recuperative furnaces and more particularly to soaking pits for heating steel ingots in which the recuperator is made of a plurality of refractory ceramic tiles laid up dry. The reason for laying them up is that provision must be made for expansion. If the tiles are cemented together to make the joints gas tight cracks, either in the tiles or joints, will result due to expansion and contraction so that. the end result is the same as if they were laid upfdry. In both cases air leakage occursinto the inside passages of the tile which carry the flue gas from the air passages formed between the tiles. Recuperators of this general type are shown in Morton et al. US. Pat. No. 2,092,402 dated Sept. 7, 1937. Air is blown through the recuperators under pressure and passes into the combustion and heating chamber of the soaking pit around or in contact with. the fuel with which it mixes and burns. The products of combustion or flue gas pass from the heating chamber and through the openings in the recuperator tile; Furnaces using this type of recuperator have b'eenin use for at least 30 years and-prior to our invention problems arose due to leakage of the combustion air into the flue gas. Morton et al. U.S. Pat. No. 2,079,560 dated'May 4, l937discloses such a soaking pit. It is desirable to heat the ingots as fast as possible in order to reduce the time of the heating cycle. The furnaces of which we have knowledge are so designed that the air travels at a higher velocity than the fuel which is normally gas, such as coke oven gas. It was found that the flue gas leaving the soaking pit contained from l to 20 percent unburned fuel at maximum firing rate and it was necessary to reduce the firing rate. Even then it was impossible to burn all the fuel. Various attempts were made to correct this situation. Oxygen was added to the air, but this was not entirely satisfactory andincreased the cost of the process. The most common way of attempting to solve the problem was to increase the volume of air being fed to the recuperators. However, it was found that the flue gas contained ap proximately the same amount of unburned fuel even when the air fed to the recuperator was 50 percent more than the stoichiometric amount required for complete combustion. This condition has existed from the time this type of furnace was first used without anyone finding a solution to our knowledge.

We have found that this condition can be corrected by increasing the area of the burner and fluegas ports without loss of mixing of the air and fuel. The area of the burner port must be increased so that it is at least 75 percent (preferably at least 100 percent) of the area of the air tunnel and the area of the flue gas ports must be increased until they are at least 75 percent (preferably at least 100 percent) of the total area of the flue gas openings in the recuperator. This results in amazing improvements. In one plant where the soaking pit furnaces are being altered in this manner, there has been a saving of as much as one half or more in the amount of fuel required to heat a ton of ingot andthe productivity has increased between 50 and 100 percent. This has been done with substantially no increase in investment since it is necessary to rebuild the burner recuperative furnace having substantially higher efficiency than similar furnaces.

Another object is to provide a soaking pit which heats ingots substantially faster than similar soaking pits not utilizing our invention.

Still another object is to provide such soaking pits in which there is a substantial fuel saving.

These and other objects will be :more apparent after referring to the following specification and attached drawings, in which:

FIG. 1 is a schematic view of a soaking pit, mostly in vertical section except for the flues and stack;

FIG. 2 is a view on an enlarged scale taken on the line Il--II ofFIG'.1; t

FIG. 3 is a vertical sectional view on an enlarged scale of a portion of the recuperator;

FIG. 4 is a view taken on the line IV--IV of FIG. 3;

and FIG. 5 is an enlarged view of a tion. Y g j Referring more particularly to FIGS. 1 and 2 of the drawings, reference numeral 2 indicates a soakingpit combustion and heating chamber having a coke bottom detail of our inven- 4 with an entry or burner port 6 therein..A removable cover17 isprovided' for chamber 2. Flue gasflows from the chamber 2 through ports 8 located at opposite sides of the chamber. ltis desirable that theports 8 be provided with beveled inlets 9'to decrease-pressure drop. This is also done at theentrances to other ports. A pair of recuperators 10 are provided for receiving the flue gas. .Each of the recuperators 10 includes a number of horizontal baffles 11. In FIG. 1, the showing'of the recuperators 10 is schematic to illustrate the air and flue gas flow, and the number of baffles and flue gas passages have no relationship to the actual numbers and construction. The actual construction is shown in FIGS. 3 and 4. Each bafile '11 is part of a horizontal wall covering the the full cross section of the recuperator and is made up of a plurality of ceramic tiles l2 hav ing an octagonal outer surface and a circular central opening 13. The tiles are laid up side by side with spaces 14 between tiles; The tiles in each baffle 11 are arranged inthe same MANNER as the adjacent bafl'les. Arranged between adjacent bames 11 are a plurality of thin walled ceramic tiles 15 one between each pair of tiles 12. The tiles 15 have an octagonal outer surface smaller than that of tiles 12 and a circular central opening 16 the same size as opening 13. Thus there are a plurality of rows of tiles with each row having a plurality of tiles laid one on top of the other with the openings 13 and 16 in alignment. This provides a space 17 between the rows of tiles. Air is delivered by means of blowers 18 into the lower end of the recuperators. In order to increase the length of travel of the air, tiles 20 are inserted into some of the spaces 14 of each of the horizontal baflles 11 so that the air follows the path of travel shown by arrows in FIG. 1. Thus the baffles 11 of FIG. 1 are formed by the combination of tiles 12 and 20 and the open space shown in FIG. 1 actually consists of tiles 12 without the tiles 20 inserted in the spaces 14.

The spacesbetween the tiles 12 and the walls of the,

recuperator are also closed by tiles where necessary. The air passes from the top of the recuperators through air tunnels 22 to the burner port 6. Fuel gas, such as coke oven gas passes to the burner port 6 through a gas pipe or port 24. The construction so far described is conventional. As shown in FIGS. 1 and 5, a hood 26 is supported on the pipe 24 by means of three lugs 28 so as to provide a peripheral opening 30 for the gas. A central opening 32 is provided through the top of hood 26. The total area of the openings 30 and 32 is substantially equal to the area of the pipe 24 with the opening 32 providing only a small amount of the area so that most of the fuel gas is diverted into the air streams from the recuperators. Hoods or divertors of this general type have been used in the past on some types of burners. It will be understood that other types of recuperators maybe used. For example, the baffles 11 could be omitted. The flue gas passes downwardly through the recuperators l and generally horizontal through flues 34 to a vertical stack 36 in the usual manner.

Prior to our invention the pressure drop from the outside to the inside of the tiles 12 adjacent the top of the recuperators was approximately 0.07 of an inch of water, whereas subsequent to the change in construction, this drop decreased to approximately 0.015 inch of water. For efiicient and proper operation this drop should not exceed 0.025 inch ofwater. This is accomplished 'by increasing the area of the burner port 6 so that it is at least 75 percent (preferably at least 100 percent) of the combined transverse area of the air tunnels 22 and by increasing the total area of ports 8 so that this area is. at least 75 percent (preferably at least 100 percent) of the total transverse area of the flue gas ports (openings 16) in the recuperators.

It will be understood that the invention may be used with other types of recuperative furnaces or soaking pits. In addition tothe bottom fired soaking pit shown the invention is being used on side fired pits of the type generally shown in Morton et al. US. Pat. No. 2,414,888 dated Jan. 28, 1947. It has been used in this type of fumace equipped with two air ports and two flue gas ports. In some instances a gas burner hood has been used and in others it is omitted. The invention is also applicable to soaking pits of the type shown in Suydarn US. Pat. No. 3,198,855 dated Apr. 24, 1962 and Mawhinney US. Pat. No. 1,915,470 dated June 27, 1933. It may also be used with furnaces having recuperators for heating blast furnace gas as well as air as shown in Morton et al. US. Pat. No. 2,124,888 dated July 26, 1938. The terms recuperator and air tunnel as used in the claims refer to a single recuperator or to any number of individual recuperators arranged in parallel, it being obvious that it is immaterial whether the series of passages are arranged in one unit or in a plurality of units. The term entry or burner port as used in the claims and the term flue gas port as used in the claims refers to either single or multiple ports, it being obvious that it is the total port area which is important. I

In operation, with the pit empty and the temperature control set at 2,400 F. the cover 7 of the pit is removed and the ingots S placed in the pit, after which the cover is replaced. Normally the steel is 3 hours old and has an outside temperature of approximately 1,400 E, with the inside of the to being considerably hotter. After charging, the temperature of the pit is about 1,600" F. As soon as the cover is closed the fuel comes on full which is approximately between 50,000 and 60,000 cu. ft. of coke oven gas per hour or 27.5 to 33 million BTU per hour. Prior to our invention only approximately 30,000 to 35,000 cu. ft. per hour could be fired. The fuel is fired at this rate for approximately 20 minutes at which time the pit temperature reaches control point and the fuel is reduced to hold the surface temperature of the ingot at the desired level while waiting for the center of the ingot to come up to rolling temperature.

Previously, it took 105 minutes to reach this temperature. The heating then continues for 1% hours with the fuel continuing to be cut until it is at a minimum of approximately 6,000 cu. ft. per hour which is sufficient to hold the pit temperature at control point. There is no change in this part of the operation. At this time the soaking period of 2 hours begins and after 2 hours the ingots are removed as they are needed by the rolling mill. The total time required is 230 minutes, as compared to 315 minutes prior to our modification. In actual practice, the ingots may be held more than 3 hours before charging into the pit and in some instances the ingots may be at ambient temperature. We have found that the savings in fuel and heating time increases as the charging temperatureof the ingots lowers with most of the savings being in the initial heating at the maximum firing 'rate. For ingots at ambient temperature it sq. in. increasing the top diameter of the air port from approximately 33 in. to 43% in., increasing the diameter of the fuel pipe to approximately 6 in. to enable more gas to flow with the existing gas pressure, and adding the hood to divert the fuel gas outwardly into the air from the recuperators. It will be understood that the figures given will vary from furnace to furnace since the air port is lined with a plastic refractory and the flue gas ports are made from refractory brick so'that the exact dimensions are dependent upon the skill of the workman.

In three pits of the type described the BTU consumed per ton of ingot averaged approximately 1,122,000; 912,000; and 791,000 before conversion and 451,000; 423,000; and 398,000 after conversion. In the same pits the ingot tons per hour averaged 13. 19, 11.2, and 13.2 before conversion and 26.53, 18.6 and 19.5 after conversion. Two pits of the side fired type were converted by increasing the diameter of the air or burner ports from 15 to 18 in. and increasing the flue gas port area from 12 to 18 sq. ft. For these furnaces the BTU consumed per ton of ingot averaged approximately 1,226,000 and 1,878,000 before conversion and 476,000 and 476,000 after conversion. In the same pits the ingot tons per hour averaged 9.1 1 and 9 before conversion and 16.4 and 17.2 after conversion.

We claim:

1. In a furnace having a combustion chamber, a recuperator formed of sets of ceramic tiles arranged end to end with aligned holes in each set of tiles forming one series of passages and openings between the sets of tiles forming a second series of passages, an entry port to said combustion chamber for receiving air from said recuperators and fuel, means for delivering air to one end of one of said series of passages, an air tunnel connecting the other end of said air passages to said entry port, and a flue gas port for receiving burnt gases from said combustion chamber and delivering it to the other of said passages; the improvement comprising an entry port area at least approximately 75 percent of the area of said air tunnel, and a flue gas port area at least approximately 75 percent of the total area of said flue gas passages.

2. A furnace according to claim 1 in which the pressure drop from the air passages to the flue gas passages at the chamber end of the recuperator is a maximum of 0.025 inches of water.

3. A furnace according to claim 1 in which the entry port area is at least as great as the area of said air tunnel and the flue gas port area is at least as great as the total area of said flue gas passages.

4. A furnace according to claim 3 in which the pressure drop from the air passages to the flue gas passages at the chamber end of the recuperator is a maximum of approximately 0.025 inches of water.

5. A furnace according to claim 3 in which the pressure drop from the air passages to the flue gas passages at the chamber end of the recuperator is a-maximum of approximately 0.015 inches of water.

6. A furnace according to claim 1 in which the combustion chamber is a pit for receiving steel ingots, said entry port extends downwardly from said pit, said flue gas port is in two sections one in each of two opposed side walls of said pit, said recuperator is in two sections one connected to each of said gas port sections and said air tunnel is in two sections one extending between each recuperator and said entry port,

7. A furnace according to claim 6 in which the pressure dee from the air passages to the flue gas passages at the chamber end of the recuperator is a maximum of approximately 0.025 inches of water.

8. A furnace according to claim 7 including a vertical pipe below and in vertical alignment with said entry port, and means on the top of said pipe for directing gas outwardly at an angle from the vertical.

9. A furnace according to claim 8 in which the entry port area is at least as great as the area of said air tunnel and the flue gas port area is at least as great as the total area of said flue gas passages.

10. A furnace according to claim 9 in which the pressure drop from the air passages to the flue gas passages at the chamber end of the recuperator is a maximum of approximately 0.015 inches of water. 

1. In a furnace having a combustion chamber, a recuperator formed of seTs of ceramic tiles arranged end to end with aligned holes in each set of tiles forming one series of passages and openings between the sets of tiles forming a second series of passages, an entry port to said combustion chamber for receiving air from said recuperators and fuel, means for delivering air to one end of one of said series of passages, an air tunnel connecting the other end of said air passages to said entry port, and a flue gas port for receiving burnt gases from said combustion chamber and delivering it to the other of said passages; the improvement comprising an entry port area at least approximately 75 percent of the area of said air tunnel, and a flue gas port area at least approximately 75 percent of the total area of said flue gas passages.
 2. A furnace according to claim 1 in which the pressure drop from the air passages to the flue gas passages at the chamber end of the recuperator is a maximum of 0.025 inches of water.
 3. A furnace according to claim 1 in which the entry port area is at least as great as the area of said air tunnel and the flue gas port area is at least as great as the total area of said flue gas passages.
 4. A furnace according to claim 3 in which the pressure drop from the air passages to the flue gas passages at the chamber end of the recuperator is a maximum of approximately 0.025 inches of water.
 5. A furnace according to claim 3 in which the pressure drop from the air passages to the flue gas passages at the chamber end of the recuperator is a maximum of approximately 0.015 inches of water.
 6. A furnace according to claim 1 in which the combustion chamber is a pit for receiving steel ingots, said entry port extends downwardly from said pit, said flue gas port is in two sections one in each of two opposed side walls of said pit, said recuperator is in two sections one connected to each of said gas port sections and said air tunnel is in two sections one extending between each recuperator and said entry port.
 7. A furnace according to claim 6 in which the pressure dee from the air passages to the flue gas passages at the chamber end of the recuperator is a maximum of approximately 0.025 inches of water.
 8. A furnace according to claim 7 including a vertical pipe below and in vertical alignment with said entry port, and means on the top of said pipe for directing gas outwardly at an angle from the vertical.
 9. A furnace according to claim 8 in which the entry port area is at least as great as the area of said air tunnel and the flue gas port area is at least as great as the total area of said flue gas passages.
 10. A furnace according to claim 9 in which the pressure drop from the air passages to the flue gas passages at the chamber end of the recuperator is a maximum of approximately 0.015 inches of water. 